Method of reducing fine iron ore in a fluidized-solids reactor



United States Patent() 3,126,276 METHOD OF REDUCING FINE IRON ORE IN AFLUIDIZED-SOLIDS REACTOR William E. Marshall, Middletown, and Arthur P.Kerschbaum, Butler County, Ohio, assignors to Armco Steel Corporation,Middletown, Ohio, a corporation of Ohio Filed Aug. 9, 1960, Ser. No.48,445 4 Claims. (Cl. 75--26) This invention relates to the reduction offine iron ores to finely divided iron by means of reducing gases underheat, where the finely divided iron ore is fluidized in the presence ofthe reducing gas. Various procedures for this purpose have hitherto beensuggested, including procedures in which the reducing gases aremaintained in a circulating condition in a closed system which includesthe reactor or fiuidizing apparatus. The gases circulating in the closedsystem have to be continuously renewed as to their reducing potential.This is accomplished in part by treatment of the gases within the closedsystem; but it also requires the introduction of fresh gas from anoutside source.

In the operation of a fluidized-solids reactor for reducing iron ore, itis customary to prepare the processing gas from natural gas in variousways, as by steammethane reforming, or partial combustion with oxygen,followed by catalytic gas shifting and carbon dioxide removal, so as toobtain gases of the desired compositions. Natural gas cannot be used forreduction without pretreatment; and any method of reforming the gasresults in the production of substantial quantities of moisture-vapor.Moist gases tend to be oxidizing rather than reducing; and therefore agas drier must be provided outside the closed recirculatory system. Thereforming of the gases is for the purpose of stepping up their reducingpotential, usually by increasing their free hydrogen content, althoughthe carbon monoxide content is also frequently increased. But it will beevident that prior proposals have required, in addition to the gasdrier, a gas shifter or reformer, and usually a carbon dioxide remover,outside the closed recirculatory system. Since these pieces of apparatushave usually been duplicated inside the closed system, it will beevident that the installation becomes cumbersome and expensive.

It is an object of the invention to provide a method and apparatusrequiring a minimum of gas-treating devices outside the closedrecirculatory system.

It is an object of the invention also to simplify the requirements forgas-treating apparatus within the closed system, by eliminating the needfor a gas shifter, and if desired, the need for a gas heater or a carbondioxide remover.

When natural gas or other gaseous fuel is cracked or otherwise fittedfor reducing use by partial combustion with air, considerable quantitiesof nitrogen dilute the active gases and form a load on the system. Theefficiency of reduction is cut down; and the cost of heating and coolingneutral gases becomes an added expense. It is an object of the inventionto provide a method of fitting natural gas or other gaseous fuels forreducing use while avoiding the accumulation and treatment of inertgases.

It is an object of the invention to provide a system in which sensibleheat from the cracking or reforming of processing gases is directlyapplied to the heating of the gases to reducing temperature.

These and other objects of the invention, which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications are accomplished by that procedure and in thatapparatus 3,126,276 Patented Mar. 24, 1964 of which an exemplaryembodiment will now be described. Reference is made to the accompanyingdrawings in which:

The drawing is a diagrammatic showing of the apparatus which is employedin practicing the invention.

Briefly, in the practice of the invention, gaseous fuel is prepared foruse as processing gas by partial combustion with oxygen at or adjacentto the fluidized-solids reactor. It will be understood that any mobilefuel may be used such as gas, oil, or pulverized coal. Examples of gasare natural gas, essentially methane, and other gases such as ethane,propane, butane and the like, providing they are available in sufiicientquantity at a low enough price. Oil can be any fuel oil. Coal can be ofany type which after pulverization can be burned with equipment commonlyused for this purpose as in steam power plants.

Natural gas is widely available and in some sections of the country isless expensive compared to other local fuels. For simplicity, naturalgas will be used in de, scribing examples given in this disclosure butit is understood that other mobile fuel can be used.

Natural gas can be subjected to partial combustion with oxygen to yieldproducts consisting essentially of hydrogen and carbon monoxide withlesser amounts of carbon dioxide, water vapor and methane. In order toaccomplish this, the gas/oxygen ratio must be kept high duringcombustion as will be understood by the skilled worker. For example,with a natural gas consisting essentially of methane, the gas/oxygenratio may be in the range of 1.2 to 1.8, with the specific ratioactually used, within this range, depending on other pro ceduresemployed in the operation.

The partial combustion generates flame temperatures between 2000 F. and3500 F. depending on the ratio used. The flame temperature is inverselyproportional to the gas oxygen ratio. The combustion chamber temperatureis, of course, lower than the flame temperature and is dependent onconstruction, size, and quantity of gas used per unit time on a specificsize of chamber.

These temperatures are too high for the direct treatment of finelydivided ores, since in most instances sticking or sintering would beencountered. Also, the products of combustion contain too high apercentage of Water vapor and carbon dioxide to be an etficient reducingmedium. For instance, the analysis of the combustion products for tworatios would be about as follows as indicated by actual combustiontests.

Analysis of Combustion Products O/S=oxygen fraction.

C/S =carbon fraction.

The 0/8 and C/S fractions are the coordinates of a point on the Gurrydiagram. The vertical distance from this point to the 1400 F. line istaken as a relative measure of the reducing potential. The Gurry diagramis widely accepted in the field of gas chemistry and is to be found inTransactions A.I.M.E., vol. 188, April 1950, page 685, FIGURE 7.

The reducing power of these gases can be increased by quenching withcold water to remove the moisture but this would sacrifice theirsensible heat which it is desired to use to heat the iron ore.Consequently, instead of subjecting the products of combustion to adrying operating, the present invention contemplates that they beimmediately mixed with cool dry reducing gases from Case ACO not removedfrom recirculated gas.

Case BC removed from recirculated gas.

Case CCO removed and recirculated gas heated to In each case the amountof recirculated gas is the quantity which after mixing with thecombustion gas will give a temperature of 1600 F. entering the reducer.

Analysis of Gas After Mixing 3g; Reduc- Case G/O H1 00 C0; 1120 CH4 0/80/8 ing Ratio circ Poten- Gas tlal A.--" 1.65 26 53 29 6 7 .195 .180.131 B--. 1.27 49 57 26 2 8 7 .160 .150 .133 C..- 1.27 60 59 25 1 7 8146 .147 .145

The Gurry diagram shows that the gas in case A has reducing poweressentially equal to that of the gas in case B where the recirculatedgas was scrubbed to remove CO Such gases are suitable for the reductionof finely divided iron ores and the excess heat generated in the flameis directly used to reheat the recirculated gases making it unnecessaryto include a reheater in the closed system. If, of course,the'recirculated gas were heated as in case C, the reducing power wouldbe increased. But the purpose here is to show that it should be possibleto operate the process without heating the recirculated gas.

The burning of the gaseous fuel with oxygen insures the absence from theprocessing gases of inert gases such as nitrogen from the air. As willbe shown later, such quantities of inert or non-reducing gases as may becontained in or derived from the gaseous fuel do not tend to accumulatein the system. The burning of the fuel with oxygen, moreover, insures ahigh temperature of the flame, at which temperature the products ofcombustion will be low in methane and in carbon dioxide.

Referring now to the drawing, the numeral 1 indicates a fluidized-solidsreactor having four stages or platforms. This is exemplary merely offluidizing devices which can be employed. Instead of a single four-stageunit, a plurality of single stage fluidizers, either in parallel or inseries, may be employed; and under some circumstances devices acting onthe cyclone separator principle are likewise available. The point isthat the processing gases shall be brought into intimate contact withthe finely divided iron ore, while both are at the proper temperaturefor reduction, in a suitable reactor. It will be understood that thereactor will be provided with means for the introduction of the iron orein finely divided condition, and means for the withdrawal of the reducedmaterial. Normally, separators are employed in connection withfluidized-solids reactors for the purpose of separating and returningfines. These last mentioned elements have not been illustrated sincethey are well known in the art.

The top gases from the reactor are carried through a conduit 2 to a gaswasher and cooler 3. In the system of this invention, gases will be bledout of the closed recirculatory system in an amount equal to the amountof fresh gases introduced. If the withdrawn gases can be used elsewhereat high temperature this may be done ahead of the washer and cooler 3.If not, it may be done 4. following the washer and cooler; and an outletfor gases has been indicated at 4. The gases bled out of the systemcontain combustible values, and hence are of use elsewhere for heating,for the generation of steam, for the protection of reduced materialtaken out of the reactor, and the like.

The gas washer and cooler 3 may take various forms which do notconstitute a limitation on the invention. Usually and preferably it isin the form of a gas washer in which the cooling medium is water. Itwill be understood that the washer and cooler is also a drying devicesince the lowering of the temperature of the gases will result in thecondensation of moisture therefrom. In the exemplary procedure the gasesmay enter the cooler and washer 3 at a temperature of 400-l000 F. and amoisture content of 10-20%, and leave it at a temperature of 6070 F. anda moisture content of about 1%.

The washer and cooler are shown connected by a conduit 5 to a pump 6;and it is one of the advantages of the system of this invention that thereduction can be carried on under pressure so that its efiiciency isincreased. The pressures normally employed in the closed recirculatorysystem are 25 p.s.i. to p.s.i.

The pump 6 sends the processing gases back to the reactor through aconduit 7. If desired a carbon dioxide absorber 8 may be connected inthis conduit and provided with a valved bypass 9. It will be understoodthat the removal of carbon dioxide will increase the reducing potentialof the processing gases; but because of the characteristics of theprocess a carbon dioxide absorber is not necessary. Thus in theexemplary procedure 1 volume of recirculated gas containing about 11% COwill be mixed with 3 volumes of fresh combustion gases, containing about4% CO formed by the partial combustion of the gaseous fuel as shown inthe aforementioned case II, to give a mixed gas in the reactorcontaining about 6% CO which is tolerable at the temperatures set forthabove. The carbon dioxide absorber if used may be of the known typeusing monoethanolamine, which will be regenerated.

The use of oxygen for the partial combustion implies the availability ofa source of this gas. Usually the oxygen will be produced in an airreduction plant which is indicated at 10 in the figure. The gases bledoff at 4 may if desired provided some of the power required in theoxygen plant.

A burner is indicated at 11 fed with oxygen and the gaseous fuel, e.g.methane in the proper proportions as previously set forth. The flamefrom the burner is formed in a combustion chamber 12 which is separatedfrom the reactor 1 by a refractory partition 13 having a centralpassageway 14 and a branch passageway 15 to which the conduit 7 isconnected. A mixture of the recirculated processing gas and the productsof combustion is thus effected before the gases enter the reactor 1; andit is desirable to provided a plenum chamber 16 at the base of thereactor for gas distribution and homogenization. Thus the gaseousmixture contacting the ore in the reactor will be of the propercomposition and at the correct temperature for efiicient reduction.

The iron ore may be finely divided oxide of iron from any source. Thereare differences in iron ores having to do with their content of silicaor other diluents. The reduced products of the iron ores differ also inaccordance with the nature of the source. Some reduced iron ores respondreadily to magnetic separation, while others do not. The reducedmaterials withdrawn from the reactor 1 will normally be protected byreducing gases (which may be some portion of the gases withdrawn at 4),until they have cooled to below the reoxidation temperature. Magneticseparation may be practiced, if feasible, on the cooled product. Thecooled reduced material may be briquetted, and will usually be melted ina suitable type of furnace such as an electric-arc furnace. Thesematters are Well known in the art and form no limitation upon thepresent invention, nor have they been illustrated.

However, it does not constitute a departure from the principles of thisinvention to employ finely divided oxides or ores in admixture withother substances, if desired. A flux such as limestone may be added tothe ores before introduction into the reactor. The iron particles formedby the reduction of some ores exhibit a greater tendency toward stickingtogether during reduc tion than do the reduced products of other ores.Fluxes are not generally effective in inhibiting sticking or sinteringduring fluidization; but finely divided carbon is effective for thispurpose. Some carbon will be produced by the cracking of the gaseousfuel; and in the process of this invention sticking may be obviated bycontrolling the amount of carbon entrained in or formed from thereducing gases by varying the ratio of gas to oxygen in the partialcombustion step. It will be understood that carbon is produced also bythe decomposition of two carbon monoxide molecules to form free carbonand a carbon dioxide molecule. This reaction tends to be more rapid attemperatures lower than the temperatures preferred for the reducinggases in the process of this invention; but the amount of carbon presentwith the iron in the reactor can be controlled also by a control oftemperature and by control of the presence of carbon monoxide in thegases.

If desired, and for the purpose of boosting the temperature of the oreand the gases in the upper part of a tall reactor, use may be made of asupplementary burner indicated at 17 in the figure, at which the gaseousfuel is burned with oxygen in the general ratios set forth above. Insome instances the burner 17 may feed its flame directly into thereactor or the arrangement shown and described for the lower burner maybe employed at the position 17, including the introduction and admixtureof processing gases from the closed recirculatory system.

The entire foregoing description is intended to be taken as an exampleof the practice of the invention in the best forms known to theinventors.

Modifications may be made in the invention without departing from thespirit of it. The invention having been described in certain exemplaryembodiments, what is claimed as new and desired to be secured by LettersPatent is:

1. In a process of reducing finely divided iron ore by fluidized-solidsreduction, the steps of:

(a) reducing fluidized finely divided iron ore by means of a reducinggaseous mixture at elevated tempera ture,

(b) withdrawing and introducing into a circulatory system all of thegases derived from said reduction step,

(c) subjecting said withdrawn gases in said circulatory system tocooling for the lowering of the moisture content of said gases, wherebyto increase their reducing potential,

(d) bleeding out of said closed system an amount of the said withdrawngases therein substantially equal to the amount of gases produced by thecombustion of fuel as hereinafter set forth,

(e) burning a mobile fuel with oxygen in the substantial absence ofinert gases so as to produce products of combustion consistingessentially of carbon monoxide, carbon dioxide, hydrogen and moisture,said products of combustion as formed being at a temperature ofsubstantially 2000 to substantially 3500 (f) mixing the cooled and driedgases in said circulating system with said products of combustion insuch quantity as to form said gaseous mixture having a temperature ofsubstantially 1500 to 1700 F., said gaseous mixture having a diminishedmoisture content as respects said products of combustion and havingstrongly reducing characteristics at said temperatures, and

(g) employing the said gaseous mixture so formed in the saidfluidized-solids reduction step.

2. The process claimed in claim 1 including the step of removing carbondioxide from the withdrawn gases in the circulatory system.

3. The process claimed in claim 1 wherein the prodnets of combustion andthe circulated gases are mixed outside the zone in which the reductionstep takes place.

4. The process claimed in claim 3 including the step of burning anadditional quantity of hydrocarbon gas with oxygen and introducing theproducts of combustion thereof into another portion of the Zone in whichthe reduction takes place.

References Cited in the file of this patent UNITED STATES PATENTS2,473,795 Hills et al. June 21, 1949 2,481,217 Hemrninger Sept. 6, 19492,547,685 Brassert et al Apr. 3, 1951 2,577,730 Benedict et al. Dec. 11,1951 2,648,535 Ramsay et a1 Aug. 11, 1953 2,740,706 Paull et al Apr. 3,1956 2,921,848 Agarwal Jan. 19, 1960

1. IN A PROCESS OF REDUCING FINELY DIVIDED IRON ORE BY FLUIDIZED-SOLIDS REDUCTION, THE STEPS OF: (A) REDUCING FLUIDIZED FINELY DIVIDED IRON ORE BY MEANS OF REDUCING GASEOUS MIXTURE AT ELEVATED TEMPERATURE, (B) WITHDRAWING AND INTRODUCING INTO A CIRCULATORY SYSTEM ALL OF THE GASES DERIVED FROM SAID REDUCTION STEP, (C) SUBJECTING SAID WITHDRAWN GASES IN SAID CIRCULATORY SYSTEM TO COOLING FOR THE LOWERING OF THE MOISTURE CONTENT OF SAID GASES, WHEREBY TO INCREASE THEIR REDUCING POTENTIAL, (D) BLEEDING OUT OF SAID CLOSED SYSTEM AN AMOUNT OF THE SAID WITHDRAWN GASES THEREIN SUBSTANTIALLY EQUAL TO THE AMOUNT OF GASES PRODUCED BY THE COMBUSTION OF FUEL AS HEREINAFTER SET FORTH, (E) BURNING A MOBILE FUEL WITH OXYGEN IN THE SUBSTANTIAL ABSENCE OF INERT GASES SO AS TO PRODUCE PRODUCTS 